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package java.net;

import java.io.IOException;
import java.util.List;
import sun.security.util.SecurityConstants;

/**
 * Selects the proxy server to use, if any, when connecting to the
 * network resource referenced by a URL. A proxy selector is a
 * concrete sub-class of this class and is registered by invoking the
 * {@link java.net.ProxySelector#setDefault setDefault} method. The
 * currently registered proxy selector can be retrieved by calling
 * {@link java.net.ProxySelector#getDefault getDefault} method.
 *
 * <p> When a proxy selector is registered, for instance, a subclass
 * of URLConnection class should call the {@link #select select}
 * method for each URL request so that the proxy selector can decide
 * if a direct, or proxied connection should be used. The {@link
 * #select select} method returns an iterator over a collection with
 * the preferred connection approach.
 *
 * <p> If a connection cannot be established to a proxy (PROXY or
 * SOCKS) servers then the caller should call the proxy selector's
 * {@link #connectFailed connectFailed} method to notify the proxy
 * selector that the proxy server is unavailable. </p>
 *
 * <P>The default proxy selector does enforce a
 * <a href="doc-files/net-properties.html#Proxies">set of System Properties</a>
 * related to proxy settings.</P>
 *
 * @author Yingxian Wang
 * @author Jean-Christophe Collet
 * @since 1.5
 */
public abstract class ProxySelector {

  /**
   * The system wide proxy selector that selects the proxy server to
   * use, if any, when connecting to a remote object referenced by
   * an URL.
   *
   * @see #setDefault(ProxySelector)
   */
  private static ProxySelector theProxySelector;

  static {
    try {
      Class<?> c = Class.forName("sun.net.spi.DefaultProxySelector");
      if (c != null && ProxySelector.class.isAssignableFrom(c)) {
        theProxySelector = (ProxySelector) c.newInstance();
      }
    } catch (Exception e) {
      theProxySelector = null;
    }
  }

  /**
   * Gets the system-wide proxy selector.
   *
   * @return the system-wide {@code ProxySelector}
   * @throws SecurityException If a security manager has been installed and it denies {@link
   * NetPermission}{@code ("getProxySelector")}
   * @see #setDefault(ProxySelector)
   * @since 1.5
   */
  public static ProxySelector getDefault() {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      sm.checkPermission(SecurityConstants.GET_PROXYSELECTOR_PERMISSION);
    }
    return theProxySelector;
  }

  /**
   * Sets (or unsets) the system-wide proxy selector.
   *
   * Note: non-standard protocol handlers may ignore this setting.
   *
   * @param ps The HTTP proxy selector, or {@code null} to unset the proxy selector.
   * @throws SecurityException If a security manager has been installed and it denies {@link
   * NetPermission}{@code ("setProxySelector")}
   * @see #getDefault()
   * @since 1.5
   */
  public static void setDefault(ProxySelector ps) {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      sm.checkPermission(SecurityConstants.SET_PROXYSELECTOR_PERMISSION);
    }
    theProxySelector = ps;
  }

  /**
   * Selects all the applicable proxies based on the protocol to
   * access the resource with and a destination address to access
   * the resource at.
   * The format of the URI is defined as follow:
   * <UL>
   * <LI>http URI for http connections</LI>
   * <LI>https URI for https connections
   * <LI>{@code socket://host:port}<br>
   * for tcp client sockets connections</LI>
   * </UL>
   *
   * @param uri The URI that a connection is required to
   * @return a List of Proxies. Each element in the the List is of type {@link java.net.Proxy
   * Proxy}; when no proxy is available, the list will contain one element of type {@link
   * java.net.Proxy Proxy} that represents a direct connection.
   * @throws IllegalArgumentException if the argument is null
   */
  public abstract List<Proxy> select(URI uri);

  /**
   * Called to indicate that a connection could not be established
   * to a proxy/socks server. An implementation of this method can
   * temporarily remove the proxies or reorder the sequence of
   * proxies returned by {@link #select(URI)}, using the address
   * and the IOException caught when trying to connect.
   *
   * @param uri The URI that the proxy at sa failed to serve.
   * @param sa The socket address of the proxy/SOCKS server
   * @param ioe The I/O exception thrown when the connect failed.
   * @throws IllegalArgumentException if either argument is null
   */
  public abstract void connectFailed(URI uri, SocketAddress sa, IOException ioe);
}
